Signal skew is a well known characteristic in high speed communications and video signal transmission. Signal skew also occurs in multiple twisted pair cables that are prevalent today in computer networking. Generally speaking, skew is the mismatch in arrival times of data on different signal lines where the data was originally transmitted at the same time. Skew is caused by different propagation rates through different pairs of cable. This, in turn, is typically caused by different twist rates for the pairs of signal lines. Paired signal wires that have a tighter twist rate cause the signals to propagate over a greater distance. Cables containing twisted pair wires are intentionally designed so that different pairs have different twist rates in order to reduce the cross talk between signal wire pairs.
In digital video monitor applications, analog video signals are transmitted by a computer over twisted pair cables to the video monitor. Usually, video signals transmitted by the computer are analog video signals broken down into the color components of the video signal. Typically, for additive color displays, analog video signals are broken down into red, green and blue color components. The red, green, blue (RGB) video signals each comprise sub-pixel data for a particular pixel. As will be illustrated, each color component of a typical analog video signal can be broken down into a series of consecutive pixel times. A pixel time is simply the period of time which elapses from the beginning of one pixel to the beginning of the next pixel. This value is not to be confused with the number of pixels, which in this context corresponds to the number of active pixels being processed. During transmission from a computer, the RGB video signals are transmitted on different sets of twisted pair signal wires.
In computer applications, the most common twisted pair cables used today are quad UTP (Unshielded Twisted Pair) cables rated as CAT5E, CAT6, etc. When video signals are transmitted on these types of UTP cables, the video sub-pixels arrive out of phase due to cable skew. For long cables, the skew error can be several pixel times. If these sub-pixels were presented on a video monitor, the display quality would be very poor due to their physical misalignment on the video monitor. This physical misalignment is directly attributable to the magnitude of the skew associated with the sub-pixels. The skew associated with the video signals must be removed in order to display the correct composite video signal on a video monitor.
One known method of compensating for signal skew involves the addition of physical delay to the earlier-arriving sub-pixels. Physical delay requires delay elements, such as an LC circuit, or a serpentine delay line. However, the use of physical delay to correct the entire skew has some undesirable characteristics. Typically, physical delay elements designed to compensate for a large amount of skew are physically large and may limit the bandwidth of the signal being passed through them. These physical delay elements may also require a complex control mechanism in order to match the delay to the skew associated with a particular cable installation. As a result of these problems, these physical delay elements are costly.
The present invention contemplates a different approach to compensating for sub-pixel signal skew. The present invention does not rely solely on physical delay to compensate for skew. However, as will be described in detail below, a small amount of physical delay may be used in conjunction with other aspects of the present invention in order to eliminate skew.
In many applications today, UTP cables are used to transmit analog video signals from a computer to a digital display monitor. The digital display monitors convert the analog video signals into digital signals using some form of an analog-to-digital (A/D) converter (or ADC).
One general method of A/D conversion is to sample the incoming analog signal at a very high rate (oversample), and then digitally process the sampled signal to obtain the digital signal information. However, at present computer video rates, this method becomes prohibitively expensive to use for digital displays due to the high speeds inherent in processing these signals. As a result, digital monitors typically sample the analog video stream at the same rate that it was created, that is, synchronous sampling. For synchronous sampling, a sampling clock must be recreated from the information contained in the received video signal. Commonly, a phase-locked loop (PLL) circuit is utilized with the phase of the clock generated by the PLL locked to the received horizontal synchronizing (Hsync) signal. The frequency of the generated clock is generally set to the number of pixel times, not the number of pixels, per scan line.
In these conventional digital display monitors, the A/D converter circuitry ideally samples the incoming analog video signal at the point where the video signal is most stable. These digital monitors use a single clock at the same phase to sample all three of the RGB analog video signals. Typically, this sample time should be around the center of each pixel, rather than near the edge of the pixel, in order to have a good low noise ratio and to achieve a representative display result. There are at least two ways in which these digital monitors adjust the sample time in order to achieve the best result. Early digital monitors included a manual adjustment mechanism. In these monitors, the user would manually adjust the sampling time until the highest quality picture was achieved. Current monitors include signal processing circuitry and features which allow automatic phase adjustment (often with a manual override for users who want to adjust the monitor display themselves). This automatic phase adjustment is implemented using closed looped feedback circuitry and signal processing techniques that are conventionally known and implemented in the art. A common form of one such technique applies different values of phase adjustment to the incoming signal. The quality of the image is checked after each application, and when all phase adjust values have been tested, the center of the largest good quality sample region—that is, the one with the least noise—is selected as the correct phase adjust value. Conventional phase adjustment techniques are described in U.S. Pat. Nos. 6,140,881; 6,597,370, and 6,522,365.
These conventional digital display monitors are usually connected to a computer through a relatively short UTP cable. Consequently, the resulting pixel skew is typically less than one pixel time. Because the pixel skew is short, a satisfactory display is achieved by adjusting the phase of the single sampling clock in the ADC of the digital monitor.
A significant problem arises, however, when the UTP cable separating the video monitor from the video source is lengthy. In these circumstances, the pixel skew can be greater than one pixel period. As a result, no adjustment to the phase of a single sampling clock can eliminate or compensate for all of the skew in the analog video signals received by the video monitor. The present invention compensates for such large scale skew without requiring the use of physical delay elements. Thus, significant advantages are achieved over the conventional methods of skew compensation.
At the highest level, one can think of the present invention as a two step approach to compensating for pixel skew. In the first step, the received video signal pixels are essentially time shifted so that the beginning and ending of each sub-pixel is aligned with the beginning and ending of the other sub-pixels. In other words, at the end of the first step of skew compensation, the various components of the received video signal (for example, red, green and blue) have their pixel edges aligned with one another. In the second stage of skew compensation, the pixels are further time shifted so that, for example, the red pixel corresponding to time period one is aligned with the green and blue sub-pixels corresponding to time period one. At the output of the second stage, digital representations of the received analog video signals are generated without skew.
To put it another way, the present invention is a two step skew compensation apparatus and method wherein the first step is an intrapixel skew compensation step and the second step is an interpixel skew compensation step. As used herein, the term “intrapixel skew” is the separation between the beginning of a pixel time for one color component and the beginning of a pixel time for a second color component, where the two pixel times at least partially overlap each other. The term “interpixel skew” is the separation between the beginning of a pixel time for one color component and the beginning of a pixel time for a second color component, where the two pixel times do not overlap each other. Thus, the distinction between intrapixel skew and interpixel skew lies in the amount of overlap of pixel times for first and second color components.